Coalescing agent for three-dimensional (3D) printing

ABSTRACT

A coalescing agent for three-dimensional (3D) printing includes a co-solvent, a surfactant having a hydrophilic lipophilic balance (HLB) value that is less than 10, a carbon black pigment, a polymeric dispersant, and a balance of water. The cosolvent is present in an amount ranging from about 15 wt % to about 30 wt % of a total wt % of the coalescing agent. The surfactant is present in an amount ranging from about 0.5 wt % to about 1.4 wt % of the total wt % of the coalescing agent. The carbon black pigment is present in an amount ranging from about 3.0 wt % to about 6.0 wt % of the total wt % of the coalescing agent. The polymeric dispersant has a weight average molecular weight ranging from about 12,000 to about 20,000.

BACKGROUND

Three-dimensional (3D) printing is an additive printing process used tomake three-dimensional solid objects from a digital model. 3D printingis often used in rapid product prototyping, mold generation, and moldmaster generation. 3D printing techniques are considered additiveprocesses because they involve the combined application of successivelayers of material. This is unlike traditional machining processes,which often rely upon the removal of material to create the finalobject. Materials used in 3D printing often require curing or fusing,which for some materials may be accomplished using heat-assistedextrusion or sintering, and for other materials may be accomplishedusing digital light projection technology.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of examples of the present disclosure willbecome apparent by reference to the following detailed description anddrawings, in which like reference numerals correspond to similar, thoughperhaps not identical, components. For the sake of brevity, referencenumerals or features having a previously described function may or maynot be described in connection with other drawings in which they appear.

FIG. 1 is a flow diagram illustrating an example of a 3D printing methoddisclosed herein;

FIGS. 2A through 2D are cross-sectional views of the steps involved informing one layer of a 3D object using an example of the 3D printingmethod disclosed herein;

FIG. 2E is a cross-sectional view of an example of the 3D object thatmay be formed after performing the steps of FIGS. 2A through 2D severaltimes;

FIG. 3 is a perspective view of the 3D object of FIG. 2E; and

FIG. 4 is a simplified isometric view of an example of a 3D printingsystem that may be used in an example of the 3D printing method asdisclosed herein.

DETAILED DESCRIPTION

Examples of the coalescing agent disclosed herein are utilized in athree-dimensional (3D) printing system. This 3D printing system is basedon a 3D fabrication process that involves the use of electromagneticradiation for fusing a build material using the inkjet appliedcoalescing agent(s) to selectively define the 3D object/part in question(layer by layer). During this 3D fabrication process, an entire layer ofa build material (e.g. a polyamide material or another suitable polymer)is exposed to the electromagnetic radiation, but only a selected regionof the build material is fused and hardened to become a layer of the 3Dobject. The coalescing agent is selectively applied by an inkjetapplicator so that the coalescing agent is in contact with the buildmaterial at the selected region. The coalescing agent includes apolymerically dispersed carbon black pigment, which improves theradiation absorbance efficiency at the selected region. Thepolymerically dispersed carbon black pigment is capable of convertingthe absorbed radiation to thermal energy, which in turn melts and/orsinters the build material that is in contact with the polymericallydispersed carbon black pigment. This causes the build material to fuseto form the layer of the 3D object.

In addition, the coalescing agent includes a co-solvent and a surfactantthat enables the coalescing agent to spread relatively uniformly overthe build material, due, at least in part, to the surfactant penetratinginto the layer of the build material. The co-solvent has a boiling pointlower than 300° C., and the surfactant has a hydrophilic lipophilicbalance (HLB) less than 10. The coalescing agent, with these particularcomponents, has a lower dynamic surface tension (i.e., the coalescingagent reaches an equilibrium surface tension of 26 dynes/cm within 10milliseconds upon being applied). The lower the dynamic surface tension,the better the dot spread of the coalescing agent and the better theoptical density of the resulting 3D object. This results in a 3D objectwith improved and more uniform cosmetic properties.

Using the coalescing agent disclosed herein, the resulting 3D object hasimproved mechanical properties (e.g., tensile strength, Young's modulus,% strain at break), for example, when compared to 3D objects formed withdifferent coalescing agents including different co-solvents and anamphoteric surfactant with no designated HLB. In some examples, it isbelieved that the improved mechanical properties may be obtained evenwhen less of the coalescing agent disclosed herein is utilized.

The coalescing agent disclosed herein is aqueous based, and includes aparticular co-solvent and surfactant. As mentioned above, the aqueousnature and particular components of the coalescing agent enhance thewetting properties of the coalescing agent, even on the build material,which may be hydrophobic. This enables the polymerically dispersedcarbon black pigment within the coalescing agent to be spread moreuniformly over the build material surface.

In an example, the coalescing agent includes water (e.g., deionizedwater), the co-solvent having a boiling point less than 300° C., thesurfactant having the HLB less than 10, and a polymerically dispersedcarbon black pigment. The amount of water in the coalescing agent mayvary depending upon the amounts of the other components, but the watermakes up a balance of the coalescing agent (i.e., so a total wt % of thecoalescing agent is 100).

As mentioned above, the co-solvent has a boiling point less than 300° C.In some examples, the co-solvent has a boiling point less than 250° C.Some examples of the co-solvent include 2-Pyrrolidinone,1,5-Pentanediol, Triethylene glycol, Tetraethylene glycol,2-methyl-1,3-propanediol, 1,6-Hexanedol, and Tripropylene glycol methylether. In the examples disclosed herein, it is to be understood that thecoalescing agent includes one of the listed co-solvents alone, or two ormore of the listed co-solvents in combination, and excludes otherco-solvents. As such, if the co-solvent is 2-Pyrrolidinone, theco-solvent 2-Pyrrolidinone alone is included. In another example, if theco-solvent is a combination of 2-Pyrrolidinone and 1,5-Pentanediol,these solvents alone are included. The co-solvent may be present in thecoalescing agent in an amount ranging from about 15 wt % to about 30 wt% based on the total wt % of the coalescing agent. In an example, theco-solvent may be present in the coalescing agent in an amount of about25 wt % based on the total wt % of the coalescing agent.

Also as mentioned above, the surfactant has the HLB less than 10. Thiscomponent contributes, at least in part, to the coalescing agent havingthe low dynamic surface tension (as defined hereinabove). Any surfactanthaving the HLB less than 10 may be used. In an example, the surfactantis a self-emulsifiable surfactant based on acetylenic diol chemistry(e.g., SURFYNOL® SE-F from Air Products and Chemical Inc.). In otherexamples, the surfactant is an ethoxylated low-foam wetting agent (e.g.,SURFYNOL® 440 or SURFYNOL® CT-111 from Air Products and Chemical Inc.)or an ethoxylated wetting agent and molecular defoamer (e.g., SURFYNOL®420 from Air Products and Chemical Inc.). Still other suitablesurfactants with an HLB less than 10 include non-ionic wetting agentsand molecular defoamers (e.g., SURFYNOL® 104E from Air Products andChemical Inc.) or water-soluble, non-ionic surfactants (e.g., TERGITOL™TMN-6 from The Dow Chemical Company). A fluorosurfactant may also beadded to the surfactant having the HLB less than 10 in order to improvethe wetting of the build material. As such, in another example, thecoalescing agent includes a combination of the surfactant with the HLBless than 10 (e.g., the self-emulsifiable surfactant based on acetylenicdiol chemistry) and a non-ionic fluororsurfactant (e.g., CAPSTONE® FS-35from DuPont).

Whether a single surfactant is used or a combination of surfactants isused, the total amount of surfactant(s) in the coalescing agent mayrange from about 0.5 wt % to about 1.4 wt % based on the total wt % ofthe coalescing agent. In an example, the surfactant having the HLB lessthan 10 is included in an amount ranging from about 0.5 wt % to about1.25 wt %, and the fluorosurfactant is included in an amount rangingfrom about 0.03 wt % to about 0.10 wt %.

An anti-kogation agent may be included in the coalescing agent. Kogationrefers to the deposit of dried ink (e.g., coalescing agent) on a heatingelement of a thermal inkjet printhead. Anti-kogation agent(s) is/areincluded to assist in preventing the buildup of kogation. Examples ofsuitable anti-kogation agents include oleth-3-phosphate (e.g.,commercially available as CRODAFOS™ O3A or CRODAFOS™ N-3 acid fromCroda), or a combination of oleth-3-phosphate and a low molecular weight(e.g., <5,000) polyacrylic acid polymer (e.g., commercially available asCARBOSPERSE™ K-7028 Polyacrylate from Lubrizol). Whether a singleanti-kogation agent is used or a combination of anti-kogation agents isused, the total amount of anti-kogation agent(s) in the coalescing agentmay range from greater than 0.20 wt % to about 0.62 wt % based on thetotal wt % of the coalescing agent. In an example, the oleth-3-phosphateis included in an amount ranging from about 0.20 wt % to about 0.60 wt%, and the low molecular weight polyacrylic acid polymer is included inan amount ranging from about 0.005 wt % to about 0.015 wt %.

In the coalescing agent disclosed herein, the carbon black pigment actsas a radiation absorbing agent or active material. Examples of carbonblack pigment include those manufactured by Mitsubishi ChemicalCorporation, Japan (such as, e.g., carbon black No. 2300, No. 900,MCF88, No. 33, No. 40, No. 45, No. 52, MA7, MA8, MA100, and No. 2200B);various carbon black pigments of the RAVEN® series manufactured byColumbian Chemicals Company, Marietta, Ga., (such as, e.g., RAVEN® 5750,RAVEN® 5250, RAVEN® 5000, RAVEN® 3500, RAVEN® 1255, and RAVEN® 700);various carbon black pigments of the REGAL® series, the MOGUL® series,or the MONARCH® series manufactured by Cabot Corporation, Boston, Mass.,(such as, e.g., REGAL® 400R, REGAL® 330R, and REGAL® 660R); and variousblack pigments manufactured by Evonik Degussa Corporation, Parsippany,N.J., (such as, e.g., Color Black FW1, Color Black FW2, Color BlackFW2V, Color Black FW18, Color Black FW200, Color Black S150, Color BlackS160, Color Black S170, PRINTEX® 35, PRINTEX® U, PRINTEX® V, PRINTEX®140U, Special Black 5, Special Black 4A, and Special Black 4).

The carbon black pigment is polymerically dispersed in the coalescingagent by a polymeric dispersant having a weight average molecular weightranging from about 12,000 to about 20,000. In some of the examplesdisclosed herein, the carbon black pigment is initially in the form of awater-based pigment dispersion. The water-based pigment dispersionincludes the carbon black pigment (which is not surface treated), thepolymeric dispersant, and water (with or without a co-solvent). Whenincluded, an example of the co-solvent may be 2-pyrollidinone. Thepolymeric dispersant may be any styrene acrylate or any polyurethanehaving its weight average molecular weight ranging from about 12,000 toabout 20,000. Some commercially available examples of the styreneacrylate polymeric dispersant are JONCRYL® 671 and JONCRYL® 683 (bothavailable from BASF Corp.). Within the water-based pigment dispersion, aratio of the carbon black pigment to the polymeric dispersant rangesfrom about 3.0 to about 4.0. In an example, the ratio of the carbonblack pigment to the polymeric dispersant is about 3.6. It is believedthat the polymeric dispersant contributes to the carbon black pigmentexhibiting enhanced electromagnetic radiation absorption.

The amount of the carbon black pigment that is present in the coalescingagent ranges from about 3.0 wt % to about 6.0 wt % based on the total wt% of the coalescing agent. In other examples, the amount of the carbonblack pigment present in the coalescing agent ranges from greater than4.0 wt % up to about 6.0 wt %. It is believed that these pigmentloadings provide a balance between the coalescing agent 26 havingjetting reliability and electromagnetic radiation absorbance efficiency.When the carbon black pigment is present in the water-based pigmentdispersion, the amount of the water-based pigment dispersion that isadded to the coalescing agent may be selected so that the amount of thecarbon black pigment in the coalescing agent is within the given ranges.

The coalescing agent may also include a chelator, abiocide/anti-microbial, and/or combinations thereof. The chelator may beadded in any amount ranging from about 0.03 wt % to about 0.10 wt %based on the total wt % of the coalescing agent. An example of asuitable chelator includes TRILON® (an aminopolycarboxylate, availablefrom BASF Corp.). The biocide or antimicrobial may be added in anyamount ranging from about 0.30 wt % to about 0.40 wt % with respect tothe total weight of the coalescing agent. Examples of suitablebiocides/anti-microbials include PROXEL™ GXL (an aqueous solution of1,2-benzisothiazolin-3-one, available from Arch Chemicals, Inc.) andKORDEK™ MLK (a formaldehyde-free microbicide from The Dow Chemical Co.).

Examples of the coalescing agent disclosed herein may be used in anysuitable 3D printing method and system. An example of the 3D printingmethod 100 is depicted in FIG. 1, and an example of the printing system10 used in various steps of the method 100 is shown in FIGS. 2A through2E. It is to be understood that each of the steps of the method 100shown in FIG. 1 will be discussed in detail herein, and in someinstances, FIGS. 2A through 2E will be discussed in conjunction withFIG. 1.

As shown at reference number 102 in FIG. 1 and in FIG. 2A, an example ofthe method 100 includes applying a build material 12 using the 3Dprinting system 10. In the example shown in FIG. 2A, one layer 14 of thebuild material 12 has been applied, as will be discussed in more detailbelow.

The build material 12 may be a powder, a liquid, a paste, or a gel.Examples of the build material 12 include polymeric semi-crystallineplastic materials with a wide processing window of greater than 5° C.(i.e., the temperature range between the melting point and there-crystallization temperature). In an example, the processing windowranges from 15° C. to about 30° C.

Examples of suitable build materials 12 include polyamides,polyethylene, polyethylene terephthalate (PET), and amorphous variationsof these materials. Still other examples of suitable build materials 12include polystyrene, polyacetals, polypropylene, polycarbonate,polyester, polyurethanes, other engineering plastics, and blends of anytwo or more of the polymers listed herein. Core shell polymer particlesof these materials may also be used.

The build material 12 may have a melting point ranging from about 55° C.to about 450° C. Some specific examples of the build material 12 havingtheir melting point within this range include nylon 11, nylon 12, nylon6, nylon 8, nylon 9, nylon 66, nylon 612, nylon 812, nylon 912, etc. Asexamples, polyamide 12 (i.e., nylon 12) has a melting point of about180°, polyamide 6 (i.e., nylon 6) has a melting point of about 220°, andpolyamide 11 (i.e., nylon 11) has a melting point of about 200°.

The build material 12 may also be a modified polyamide. In an example,the modified polyamide material is an elastomeric modified polyamidethat melts at a lower temperature than nylon 12.

When the build material 12 is in powder form, the polyamide build 12 maybe made up of similarly sized particles (as shown in FIG. 2A) ordifferently sized particles. In an example, the build material 12includes particles of three different sizes. In this example, theaverage size of the first particle is larger than the average size ofthe second particle, and the average size of the second polymer particlemay be larger than the average size of the third polymer particle. Theterm “size”, as used herein, refers to the diameter of a sphericalparticle, or the average diameter of a non-spherical particle (i.e., theaverage of multiple diameters across the non-spherical particle). Ingeneral, the average size of the particles of the build material 12ranges from about 10 μm to about 100 μm. In some examples, the averagesize of the particles of the build material 12 ranges from about 40 μmto about 50 μm. As an example of the different sizes for each of theparticles, the average size of the first particle may be greater than 50μm, the average size of the second particle may be between 10 μm and 30μm, and the average size of the third particle may be equal to or lessthan 10 μm. In an example, the first polyamide particle is present in anamount ranging from about 70 wt % to about 95 wt %, the second polyamideparticle is present in an amount ranging from about 0.5 wt % to about 21wt %, and the third polyamide particle is present in an amount rangingfrom greater than 0 wt % up to about 21 wt %.

It is to be understood that build material 12 may include, in additionto build material particles, a charging agent, a flow aid, orcombinations thereof. Charging agent(s) may be added to suppresstribo-charging. Examples of suitable charging agent(s) include aliphaticamines (which may be ethoxylated), aliphatic amides, quaternary ammoniumsalts (e.g., behentrimonium chloride or cocamidopropyl betaine), estersof phosphoric acid, polyethylene glycol esters, or polyols. Somesuitable commercially available charging agents include HOSTASTAT® FA 38(natural based ethoxylated alkylamine), HOSTASTAT® FE2 (fatty acidester), and HOSTASTAT® HS 1 (alkane sulfonate), each of which isavailable from Clariant Int. Ltd.). In an example, the charging agent isadded in an amount ranging from greater than 0 wt % to less than 5 wt %based upon the total wt % of the build material particles.

Flow aid(s) may be added to improve the coating flowability of the buildmaterial 12. Flow aid(s) may be particularly desirable when theparticles are less than 25 μm in size. The flow aid improves theflowability of the build material 12 by reducing the friction, thelateral drag, and the tribocharge buildup (by increasing the particleconductivity). Examples of suitable flow aids include tricalciumphosphate (E341), powdered cellulose (E460(ii)), magnesium stearate(E470b), sodium bicarbonate (E500), sodium ferrocyanide (E535),potassium ferrocyanide (E536), calcium ferrocyanide (E538), bonephosphate (E542), sodium silicate (E550), silicon dioxide (E551),calcium silicate (E552), magnesium trisilicate (E553a), talcum powder(E553b), sodium aluminosilicate (E554), potassium aluminum silicate(E555), calcium aluminosilicate (E556), bentonite (E558), aluminumsilicate (E559), stearic acid (E570), or polydimethylsiloxane (E900). Inan example, the flow aid is added in an amount ranging from greater than0 wt % to less than 5 wt % based upon the total wt % of the buildmaterial particles.

Referring back to FIG. 2A, the printing system 10 for forming the 3Dobject includes a supply bed 16 (including a supply of the buildmaterial 12), a delivery piston 18, a roller 20, a fabrication bed 22(having a contact surface 23), and a fabrication piston 24. Each ofthese physical elements may be operatively connected to a centralprocessing unit (not shown) of the printing system 10. The centralprocessing unit (e.g., running computer readable instructions stored ona non-transitory, tangible computer readable storage medium) manipulatesand transforms data represented as physical (electronic) quantitieswithin the printer's registers and memories in order to control thephysical elements to create the 3D object. The data for the selectivedelivery of the build material 12, the coalescing agent, etc. may bederived from a model of the 3D object to be formed.

The delivery piston 18 and the fabrication piston 24 may be the sametype of piston, but are programmed to move in opposite directions. In anexample, when a first layer of the 3D object is to be formed, thedelivery piston 18 may be programmed to push a predetermined amount ofthe build material 12 out of the opening in the supply bed 16, and thefabrication piston 24 may be programmed to move in the oppositedirection of the delivery piston 18 in order to increase the depth ofthe fabrication bed 22. The delivery piston 18 will advance enough sothat when the roller 20 pushes the build material 12 into thefabrication bed 22 and onto the contact surface 23, the depth of thefabrication bed 22 is sufficient so that a layer 14 of the buildmaterial 12 may be formed in the bed 22. The roller 20 is capable ofspreading the build material 12 into the fabrication bed 22 to form thelayer 14, which is relatively uniform in thickness. In an example, thethickness of the layer 14 ranges from about 90 μm to about 110 μm,although thinner or thicker layers may also be used.

It is to be understood that the roller 20 may be replaced by othertools, such as a blade that may be desirable for spreading differenttypes of powders, or a combination of a roller and a blade.

After the layer 14 of the build material 12 is introduced into thefabrication bed 22, the layer 14 is exposed to heating (as shown atreference number 104 in FIG. 1 and in FIG. 2B). Heating is performed topre-heat the build material 12, and thus it is desirable that theheating temperature be below the melting point of the build material 12.As such, the heating temperature selected will depend upon the buildmaterial 12 that is used. As examples, the heating temperature may befrom about 5° C. to about 20° C. below the melting point of the buildmaterial 12. In an example, the build material 12 is heated to atemperature ranging from about 50° C. to about 430° C. In an example inwhich the build material 12 is polyamide 12, the pre-heating temperatureranges from about 160° C. to about 170° C.

Pre-heating the layer 14 of the build material 12 may be accomplishedusing any suitable heat source that exposes all of the build material 12in the fabrication bed 22 to the heat. Examples of the heat sourceinclude an electromagnetic radiation source, such as an infrared lightsource or near-infrared light source.

After pre-heating the layer 14, the previously described coalescingagent 26 is selectively applied on a portion of the build material 12 inthe layer 14, as shown at reference number 106 in FIG. 1 and in FIG. 2C.As illustrated in FIG. 2C, the coalescing agent 26 may be dispensed froman inkjet applicator 28 (e.g., a thermal inkjet printhead or apiezoelectric inkjet printhead). While a single inkjet applicator 28 isshown in FIG. 2C, it is to be understood that multiple inkjetapplicators may be used that span the width of the fabrication bed 22.The inkjet applicator(s) 28 may be attached to a moving XY stage or atranslational carriage (neither of which is shown) that moves the inkjetapplicator(s) 28 adjacent to the fabrication bed 22 in order to depositthe coalescing agent 26 in desirable area(s).

The inkjet applicator(s) 28 may be programmed to receive commands fromthe central processing unit and to deposit the coalescing agent 26according to a pattern of a cross-section for the layer of the 3D objectthat is to be formed. As used herein, the cross-section of the layer ofthe object to be formed refers to the cross-section that is parallel tothe contact surface 23. The inkjet applicator(s) 28 selectively appliesthe coalescing agent 26 on those portions of the layer 14 that are to befused to become one layer of the 3D object. As an example, if the firstlayer is to be shaped like a cube or cylinder, coalescing agent 26 willbe deposited in a square pattern or a circular pattern (from a topview), respectively, on at least a portion of the layer 14 of the buildmaterial 12. In the example shown in FIG. 2C, the coalescing agent 26 isdeposited in a square pattern on the area or portion 30 of the layer 14and not on the areas or portions 32.

In the example of the method 100 disclosed herein, a single coalescingagent 26 may be selectively applied to form the layer of the 3D object.It is to be understood that the amount of the coalescing agent 26 thatis applied may be adjusted digitally to change the properties in thefinal product.

After the coalescing agent(s) 26 is/are selectively applied in thedesired area(s) or portion(s) 30, the entire layer 14 of the buildmaterial 12 and the coalescing agent(s) 26 applied to at least a portionthereof are exposed to electromagnetic radiation. This is shown at step108 in FIG. 1 and in FIG. 2D.

In an example, the electromagnetic radiation may be infrared ornear-infrared radiation. The electromagnetic radiation is emitted from aradiation source 34, such as an IR or near-IR curing lamp, IR or near-IRlight emitting diodes (LED), or lasers with the desirableelectromagnetic wavelengths. In an example, the light sourceelectromagnetic wavelengths range from about 100 nm (UV) to about 10 μm.In another example, the light source is a near-infrared light sourcewith wavelengths of about 800 nm. In yet another example, the lightsource is an infrared light source with wavelengths of about 2 μm. Theradiation source 34 may be attached, for example, to a carriage thatalso holds the inkjet applicator(s) 28. The carriage may move theradiation source 34 into a position that is adjacent to the fabricationbed 22. The radiation source 34 may be programmed to receive commandsfrom the central processing unit and to expose the layer 14 and appliedcoalescing agent 26 to electromagnetic energy (e.g., IR or near-IRenergy).

The length of time the radiation is applied for, or the energy exposuretime, may be dependent, for example, on one or more of: characteristicsof the radiation source 34; characteristics of the build material 12;and/or characteristics of the coalescing agent 26.

It is to be understood that variations in the fusing level may beachieved by altering (increasing or decreasing) the energy exposure timealong the X, Y, and/or Z axes. As an example, if it is desirable thatthe level of fusing decrease along the Z axis, the radiation exposuretime may be the highest in the first layer and decrease in subsequentlyformed layers. In yet another example, variations in the level of fusingmay be achieved by altering (increasing or decreasing) the amount ofcoalescent agent 26 that is applied along the X, Y, and/or Z axes.

The coalescing agent 26 enhances the absorption of the electromagneticenergy, converts the absorbed electromagnetic to thermal energy, andpromotes the transfer of the thermal heat to the build material 12 incontact with the coalescing agent 26 (i.e., in the area(s)/portion(s)32). In an example, the coalescing agent 26 sufficiently elevates thetemperature of the build material 12 in the area(s) 32 near or above itsmelting point, allowing fusing (which may include melting, sintering,binding, etc.) of the build material 12 to take place. In a specificexample, the temperature is elevated about 50° C. above the meltingtemperature of the build material 12. The coalescing agent 26 may alsocause, for example, heating of the build material 12 below its meltingpoint but to a temperature suitable to cause softening and bonding. Itis to be understood that area(s) 32 not having the coalescing agent 26applied thereto absorb less energy, and thus the build material 12within these area(s) 32 generally does not exceed the melting point anddoes not fuse. This forms one layer 40 of the 3D object 50 (FIGS. 2E and3) to be formed.

As mentioned above, the exposure to the electromagnetic radiation fusesthe build material 12 in the area(s) 32 to form the layer 40 of the 3Dobject 50. Steps 102 through 108 of FIG. 1 may be repeated as many timesas desirable to create subsequent layers 42, 44, 46 (FIG. 2E) and toultimately form the 3D object 50. It is to be understood that heatabsorbed (during the application of energy) by a portion of the buildmaterial 12 on which coalescing agent 26 has been applied or haspenetrated may propagate to a previously solidified layer, such as layer40, causing at least some of that layer 40 to heat up above its meltingpoint. This effect helps create strong interlayer bonding betweenadjacent layers (e.g., 40 and 42) of the 3D object 50.

FIG. 2E illustrates one example of the 3D object 50 formed in thefabrication bed 22. It is to be understood that the subsequently formedlayers 42, 44, 46 may have any desirable shape and/or thickness and maybe the same as, or different from any other layer 40, 42, 44, 46,depending upon the size, shape, etc. of the 3D object 50 that is to beformed.

As illustrated in FIG. 2E, as subsequent layers 42, 44, 46 have beenformed, the delivery piston 18 is pushed closer to the opening of thedelivery bed 16, and the supply of the build material 12 in the deliverybed 16 is diminished (compared, for example, to FIG. 2A at the outset ofthe method 100). The fabrication piston 24 is pushed further away fromthe opening of the fabrication bed 22 in order to accommodate thesubsequent layer(s) of build material 12 and selectively appliedcoalescing agent 26. Since at least some of the build material 12remains unfused after each layer 40, 42, 44, 46 is formed, the 3D object50 is at least partially surrounded by the unfused build material 12 inthe fabrication bed 22.

When the 3D object 50 is complete, it may be removed from thefabrication bed 22, and the unfused build material 12 remaining in thefabrication bed 22 may be reused depending, in part, on processconditions.

FIG. 3 illustrates a perspective view of the 3D object 50. Each of thelayers 40, 42, 44, 46 includes fused (melted, sintered, bonded, etc.)build material and at least some components of the coalescing agent 26(i.e., those that have not evaporated).

Referring now to FIG. 4, another example of the printing system 10′ isdepicted. The system 10′ includes a central processing unit (CPU) 56that controls the general operation of the additive printing system 10′.As an example, the central processing unit (CPU) 56 may be amicroprocessor-based controller that is coupled to a memory 52, forexample via a communications bus (not shown). The memory 52 stores thecomputer readable instructions 54. The central processing unit 56 mayexecute the instructions 54, and thus may control operation of thesystem 10′ in accordance with the instructions 54.

In this example, the printing system 10′ includes the inkjet applicator28 to selectively deliver/apply the coalescing agent 26 to a layer 14(not shown in this figure) of build material 12 provided on a supportmember 60. In an example, the support member 60 has dimensions rangingfrom about 10 cm by 10 cm up to about 100 cm by 100 cm, although thesupport member 60 may have larger or smaller dimensions depending uponthe 3D object 50 that is to be formed.

The central processing unit 56 controls the selective delivery of thecoalescing agent 26 to the layer 14 of the build material 12 inaccordance with delivery control data 58.

In the example shown in FIG. 4, it is to be understood that the inkjetapplicator 28 is a printhead, such as a thermal printhead or apiezoelectric inkjet printhead. The inkjet applicator 28 may be adrop-on-demand printhead or a continuous drop printhead.

The inkjet applicator 28 may be used to selectively deliver thecoalescing agent 26 when in the form of a suitable fluid. As describedabove, the coalescing agent 26 includes an aqueous vehicle, such aswater, the co-solvent, the surfactant, etc., to enable it to bedelivered via the inkjet applicator 28.

In one example, the inkjet applicator 28 may be selected to deliverdrops of the coalescing agent 26 at a resolution ranging from about 300dots per inch (DPI) to about 1200 DPI. In other examples, the inkjetapplicator 28 may be selected to be able to deliver drops of thecoalescing agent 26 at a higher or lower resolution.

The inkjet applicator 28 may include an array of nozzles through whichthe inkjet applicator 28 is able to selectively eject drops of fluid. Inone example, each drop may be in the order of about 10 pico liters (pl)per drop, although it is contemplated that a higher or lower drop sizemay be used. In some examples, inkjet applicator 28 is able to delivervariable size drops.

The inkjet applicator 28 may be an integral part of the printing system10′, or it may be user replaceable. When the inkjet applicator 28 isuser replaceable, it may be removably insertable into a suitabledistributor receiver or interface module (not shown).

In another example of the printing system 10′, a single inkjet printheadmay be used to selectively deliver different coalescing agents 26. Forexample, a first set of printhead nozzles of the printhead may beconfigured to deliver one of the coalescing agents 26, and a second setof printhead nozzles of the printhead may be configured to deliver theother of the coalescing agents 26.

As shown in FIG. 4, the inkjet applicator 28 has a length that enablesit to span the whole width of the support member 60 in a page-wide arrayconfiguration. In an example, the page-wide array configuration isachieved through a suitable arrangement of multiple inkjet applicators28. In another example, the page-wide array configuration is achievedthrough a single inkjet applicator 28 with an array of nozzles having alength to enable them to span the width of the support member 60. Inother examples of the printing system 10′, the inkjet applicator 28 mayhave a shorter length that does not enable them to span the whole widthof the support member 60.

While not shown in FIG. 4, it is to be understood that the inkjetapplicator 28 may be mounted on a moveable carriage to enable it to movebi-directionally across the length of the support member 60 along theillustrated Y-axis. This enables selective delivery of the coalescingagent 26 across the whole width and length of the support member 60 in asingle pass. In other examples, the inkjet applicator 28 may be fixedwhile the support member 60 is configured to move relative thereto.

As used herein, the term ‘width’ generally denotes the shortestdimension in the plane parallel to the X and Y axes shown in FIG. 4, andthe term ‘length’ denotes the longest dimension in this plane. However,it is to be understood that in other examples the term ‘width’ may beinterchangeable with the term ‘length’. As an example, the inkjetapplicator 28 may have a length that enables it to span the whole lengthof the support member 60 while the moveable carriage may movebi-directionally across the width of the support member 60.

In examples in which the inkjet applicator 28 has a shorter length thatdoes not enable it to span the whole width of the support member 60, theinkjet applicator 28 may also be movable bi-directionally across thewidth of the support member 60 in the illustrated X axis. Thisconfiguration enables selective delivery of the coalescing agent 26across the whole width and length of the support member 60 usingmultiple passes.

The inkjet applicator 28 may include therein a supply of the coalescingagent 26, or may be operatively connected to a separate supply of thecoalescing agent 26.

As shown in FIG. 4, the printing system 10′ also includes a buildmaterial distributor 64. This distributor 64 is used to provide thelayer (e.g., layer 14) of the build material 12 on the support member60. Suitable build material distributors 64 may include, for example, awiper blade, a roller, or combinations thereof.

The build material 12 may be supplied to the build material distributor64 from a hopper or other suitable delivery system. In the exampleshown, the build material distributor 64 moves across the length (Yaxis) of the support member 60 to deposit a layer of the build material12. As previously described, a first layer of build material 12 will bedeposited on the support member 60, whereas subsequent layers of thebuild material 12 will be deposited on a previously deposited (andsolidified) layer.

It is to be further understood that the support member 60 may also bemoveable along the Z axis. In an example, the support member 60 is movedin the Z direction such that as new layers of build material 12 aredeposited, a predetermined gap is maintained between the surface of themost recently formed layer and the lower surface of the inkjetapplicator 28. In other examples, however, the support member 60 may befixed along the Z axis, and the inkjet applicator 28 may be movablealong the Z axis.

Similar to the system 10, the system 10′ also includes the radiationsource 34 to apply energy to the deposited layer of build material 12and the selectively applied coalescing agent 26 to cause thesolidification of portion(s) 32 of the build material 12. Any of thepreviously described radiation sources 34 may be used. In an example,the radiation source 34 is a single energy source that is able touniformly apply energy to the applied materials, and in another example,radiation source 34 includes an array of energy sources to uniformlyapply energy to the deposited materials.

In the examples disclosed herein, the radiation source 34 is configuredto apply energy in a substantially uniform manner to the whole surfaceof the deposited build material 12. This type of radiation source 34 maybe referred to as an unfocused energy source. Exposing the entire layerto energy simultaneously may help increase the speed at which athree-dimensional object 50 may be generated.

While not shown, it is to be understood that the radiation source 34 maybe mounted on the moveable carriage or may be in a fixed position.

The central processing unit 56 may control the radiation source 34. Theamount of energy applied may be in accordance with delivery control data58.

The system 10′ may also include a pre-heater 62 that is used to pre-heatthe deposited build material 12 (as shown and described in reference toFIG. 2B). The use of the pre-heater 62 may help reduce the amount ofenergy that has to be applied by the radiation source 34.

To further illustrate the present disclosure, examples are given herein.It is to be understood that these examples are provided for illustrativepurposes and are not to be construed as limiting the scope of thepresent disclosure.

Example 1

A coalescing agent was prepared according to the examples disclosedherein. The formulation of the example coalescing agent is shown inTable 1. A comparative coalescing agent was also prepared. Theformulation of the comparative coalescing agent is also shown inTable 1. The amounts are provided as weight percentages. The carbonblack pigments were added to the formulation in the form of an aqueousdispersion (including water and a polymeric dispersant), but the amountprovided in Table 1 represents the actual amount of the carbon blackpigment.

TABLE 1 Example and Comparative Example Coalescing agents (CAs)Comparative Component Example CA 1 Example CA 1 Type Component wt % wt %Co-solvent 2-pyrollidinone 25 15 Ethoxylated Glycerol-1 (LEG-1) 0 1.5Glycerol 0 2.5 Trimethylolpropane (EHPD) 0 2.5 1,5-Pentanediol 0 2.5Surfactant MACKAM ® CB35-ULS^(a) 0 0.1 CHEMGUARD ™ S550L-ULS^(aa) 00.012 CAPSTONE ® FS-35^(aa) 0.05 0 SURFYNOL ® SE-F^(aaa) 0.75 0Anti-Kogation CRODAFOS ™ O3A^(b) 0.5 0 Agent CARBOSPERSE ™ K-7028 0.01 0Polyacrylate^(bb) JONCRYL ® 683^(bbb) 0 0.5 Buffer Tris Buffer 0 0.2Chelator TRILON ® M^(c) 0.04 0 Biocide PROXEL ™ GXL^(d) 0.14 0.2KORDEK ™ MLK^(dd) 0.18 0 Carbon Black CB1^(e) 0 4.0 Pigment CB2^(ee) 5.00 Water Deionized water balance balance ^(a)Amphoteric Coco-Betainesurfactant from Solvay Novacare, no designated HLB value^(aa)Fluorosurfactants from Chemguard Inc. and DuPont, respectively^(aaa)Self-emulsifiable surfactant based on acetylenic diol chemistryfrom Air Products and Chemical Inc. ^(b)Oleth-3-phosphate from Croda^(bb)From Lubrizol ^(bbb)Styrene acrylate polymeric dispersant from BASFCorp. (100% active) ^(c)Aminopolycarboxylate from BASF Corp. ^(d)anaqueous solution of 1,2-benzisothiazolin-3-one available from ArchChemicals, Inc. ^(dd)a formaldehyde-free microbicide from The DowChemical Co. ^(e)CB1 was a carbon black ink dispersion from Cabot,including a surface treated carbon black pigment (not polymericallydispersed) ^(ee)CB2 was a carbon black ink dispersion fromHewlett-Packard, including a non-surface treated carbon black pigmentdispersed with JONCRYL ® 671

The example CA 1 and the comparative example CA 1 were used to form 3Dobjects with two different types of polyamide-12, namely PA2200(available from Electro Optical Systems) and VESTOSINT X1556 (availablefrom Evonik). A layer of each of the polyamide-12 (PA-12) materials wasapplied to a fabrication bed. To form an example PA2200 part, theexample CA 1 was thermal inkjet printed with a 9 ng printhead in apattern on a portion of the PA2200 layer, and was exposed to IRradiation. To form an example VESTOSINT X1556 part, the example CA 1 wasthermal inkjet printed with a 9 ng printhead in a pattern on a portionof the VESTOSINT X1556 layer, and was exposed to IR radiation. To form acomparative example PA2200 part, the comparative example CA1 was thermalinkjet printed with a 9 ng printhead in a pattern on a portion of thePA2200 layer, and was exposed to IR radiation. To form a comparativeexample VESTOSINT X1556 part, the comparative example CA 1 was thermalinkjet printed with a 9 ng printhead in a pattern on a portion of theVESTOSINT X1556 layer, and was exposed to IR radiation. Each of theexample CA 1 and the comparative example CA 1 was applied in the sameamount in the XY direction and in the Z direction. The radiationexposure was about 5 inches per second.

The example PA2200 parts, the example VESTOSINT X1556 parts, thecomparative example PA2200 parts, and the comparative example VESTOSINTX1556 parts were tested to determine various mechanical properties,including tensile strength, Young's modulus, % strain at yield, % strainat break, and % strain (break-yield). Each of the example parts and thecomparative example parts was tested in the uniaxial tension using themethod described in ASTM D638 (“Standard Test Method for TensileProperties of Plastics”). The results are shown in Table 2.

TABLE 2 Mechanical Properties of Example and Comparative Example PartsPA2200 parts VESTOSINT X1556 parts Mechanical Comparative ExampleComparative Example Properties CA 1 CA1 CA 1 CA 1 Tensile Strength (MPa)XY 44.2 45.3 37 42.3 Z 40.8 43.7 Not tested Not tested Young's Modulus(MPa) XY 1609 1631 1201 1368 Z 1371 1528 Not tested Not tested % Strainat Yield XY 0 9 9.4 11.2 Z 7.8 9.4 Not tested Not tested % Strain atBreak XY 3.7 16 10 16.7 Z 8.1 10.3 Not tested Not tested % Strain(Break-Yield) XY 0 6.8 0.6 5.5 Z 0.3 0.9 Not tested Not tested

As shown in Table 2, in the XY and the Z directions, all of themechanical properties of the example PA2200 part (made with exampleCA 1) were better than the mechanical properties of comparative examplePA2200 part (made with comparative example CA 1). As such, both the XYand the Z axis strength were improved in the example PA2200 part.

Also as shown in Table 2, in the XY direction, all of the mechanicalproperties of the example VESTOSINT X1556 part were better than themechanical properties of comparative example VESTOSINT X1556 part.

In addition, it is believed that the mechanical properties may befurther improved by adjusting the amount of the example coalescing agentthat is used.

Example 2

In order to test the effect of the pigment type used in the coalescingagent on the mechanical properties of the formed 3D object, threedifferent carbon black pigment dispersions were used to make threedifferent coalescing agents. The vehicle of the different coalescingagents was otherwise the same. It is noted that the vehicle did notinclude the surfactant having the HLB less than 10. One examplecoalescing agent was prepared (Example CA 2), and two comparativecoalescing agents were prepared (Comparative Example CA 2 and 3). Thecarbon black pigment in comparative example CA 2 was CB1 from Example 1(i.e., a surface treated carbon black pigment that was not polymericallydispersed). The carbon black pigment in comparative example CA 3 was acarbon black pigment polymerically dispersed with a styrene acrylatehaving a weight average molecular weight less than 12,000. The carbonblack pigment in the Example CA 2 was CB2 from Example 1 (i.e., a carbonblack pigment dispersed with JONCRYL® 671—a styrene acrylate having aweight average molecular weight of about 17,000). The formulations areshown in Table 3.

TABLE 3 Example and Comparative Example Coalescing agents ExampleComparative Comparative Component CA 2 Example CA 2 Example CA 3 TypeComponent wt % wt % wt % Co-solvent 2-pyrollidinone 16 16 16 EthoxylatedGlycerol-1 1.5 1.5 1.5 (LEG-1) Tetraethylene glycol 2 2 22-Methyl-1,3-Propanediol 2 2 2 Surfactant CHEMGUARD ™ S550L- 0.2 0.2 0.2ULS^(a) SURFYNOL ® CT151^(aa) 0.15 0.15 0.15 Binder SMA 1440 K^(b) 0.70.7 0.7 Buffer Tris Buffer 0.1 0.1 0.1 Carbon CB1^(c) 0 0 4.5 BlackCB3^(cc) 0 4.5 0 Pigment CB2^(ccc) 4.5 0 0 Water Deionized water balancebalance balance ^(a)Fluorosurfactants from Chemguard Inc. ^(aa)Anionicsurfactant from Air Products and Chemical Inc. ^(b)Styrene Maleicanhydride polymer from Satomer Co. ^(c)CB1 was a carbon black inkdispersion from Cabot, including a surface treated carbon black pigment(not polymerically dispersed) ^(cc)CB3 was a carbon black ink dispersionfrom DIC Corp., including carbon black pigment polymerically dispersedwith a styrene acrylate having a weight average molecular weight lessthan 12,000 ^(ccc)CB2 was a carbon black ink dispersion fromHewlett-Packard, including non-surface treated carbon black pigmentdispersed with JONCRYL ® 671

The example CA 2 and the comparative examples CA 2 and CA 3 were used toform 3D objects with polyimide-12. A layer of each of the polyimide-12(PA-12) materials was applied to a fabrication bed. To form an examplepart, the example CA 2 was thermal inkjet printed with a 9 ng printheadin a pattern on a portion of the PA-12 layer, and was exposed to IRradiation. To form comparative example PA-12 parts, the comparativeexamples CA 2 AND CA 3 were respectively thermal inkjet printed with a 9ng printhead in a pattern on a portion of the PA-12 layer, and wereexposed to IR radiation. Each of the example CA 2 and the comparativeexamples CA 2 and CA 3 was applied in the same amount in the XYdirection and in the Z direction. The radiation exposure was about 5inches per second.

The example part and the comparative example parts were tested todetermine various mechanical properties, including tensile strength,Young's modulus, and % strain at yield. Each of the example part and thecomparative example parts was tested in the uniaxial tension using themethod described in ASTM D638 (“Standard Test Method for TensileProperties of Plastics”). The results are shown in Table 4. Themechanical properties along the Z axis were not tested.

TABLE 4 Mechanical Properties of Example and Comparative Example PartsComparative Example Parts Mechanical Example Part ComparativeComparative Properties Example CA 2 CA 2 CA 3 Tensile Strength (MPa) XY43.0 39.5 41.6 Young's Modulus (MPa)* XY 562 538 549 % Strain at YieldXY 20.9 13.0 15.9 *Numbers are lower than in Example 1 because themeasurements were performed without an extensiometer

The mechanical properties of the example part formed with CB2 werebetter than either of the parts formed with the other carbon black inkdispersions (CB1 and CB3). It is believed that the mechanical propertiesof parts formed with Example CA 2 may be even further enhanced byincorporating the surfactant having an HLB value that is less than 10.

Example 3

In order to test the effect of the anti-kogation agent level used in thecoalescing agent on the mechanical properties of the formed 3D object,two different anti-kogation agent levels were used to make two differentcoalescing agents. One example coalescing agent was prepared (Example CA3), and one comparative coalescing agent was prepared (ComparativeExample CA 4). The formulations are shown in Table 5.

TABLE 5 Example and Comparative Example Coalescing agents ComparativeExample Example Component CA 3 CA 4 Type Component wt % wt % Co-solvent2-pyrollidinone 25 25 Surfactant CAPSTONE ® FS-35^(a) 0.05 0.05SURFYNOL ® SE-F^(aa) 0.75 0.75 Anti-Kogation CRODAFOS ™ O3A^(b) 0.5 0.2Agent CARBOSPERSE ™ K-7028 0.01 0.01 Polyacrylate^(bb) Chelator TRILON ®M^(c) 0.04 0.04 Biocide PROXEL ™ GXL^(d) 0.14 0.12 KORDEK ™ MLK^(dd)0.18 0.18 Carbon Black CB2^(e) 5.0 5.0 Pigment Water Deionized waterbalance balance ^(a)Fluorosurfactant from Chemguard Inc.^(aa)Self-emulsifiable surfactant based on acetylenic diol chemistryfrom Air Products and Chemical Inc. ^(b)Oleth-3-phosphate from Croda^(bb)From Lubrizol ^(c)Aminopolycarboxylate from BASF Corp. ^(d)anaqueous solution of 1,2-benzisothiazolin-3-one available from ArchChemicals, Inc. ^(dd)a formaldehyde-free microbicide from The DowChemical Co. ^(e)CB2 was a carbon black ink dispersion fromHewlett-Packard, including non-surface treated carbon black pigmentdispersed with JONCRYL ® 671

The example CA 3 and the comparative example CA 4 were used to form 3Dobjects with polyimide-12. A layer of each of the polyimide-12 (PA-12)materials was applied to a fabrication bed. To form an example part, theexample CA 3 was thermal inkjet printed with a 9 ng printhead in apattern on a portion of the PA-12 layer, and was exposed to IRradiation. To form comparative example PA-12 part, the comparativeexample CA 4 was thermal inkjet printed with a 9 ng printhead in apattern on a portion of the PA-12 layer, and was exposed to IRradiation. Each of the example CA 3 and the comparative example CA 4 wasapplied in the same amount in the XY direction and in the Z direction.The radiation exposure was about 5 inches per second.

The example part and the comparative example part were tested todetermine various mechanical properties, including tensile strength,Young's modulus, % strain at yield, % strain at break, and % strain(break-yield). Each of the example parts and the comparative exampleparts was tested in the uniaxial tension using the method described inASTM D638 (“Standard Test Method for Tensile Properties of Plastics”).The results are shown in Table 6. The mechanical properties along the Zaxis were not tested.

TABLE 6 Mechanical Properties of Example and Comparative Example PartsMechanical PA-12 parts Properties Example CA3 Comparative CA 4 TensileStrength (MPa) XY 45.3 44.8 Young's Modulus (MPa) XY 1631 1491 % Strainat Yield XY 9.2 9.0 % Strain at Break XY 16 16 % Strain (Break-Yield) XY6.8 7.1

As shown in Table 6, all of the mechanical properties, except for %strain (break-yield) of the example PA-12 part (made with example CA 3)were better than or equal to the mechanical properties of comparativeexample PA-12 part (made with comparative example CA 4). As such, theanti-kogation agent level may be included in a suitable amount in thecoalescent agent disclosed herein in order to improve the mechanicalproperties of the 3D object.

Reference throughout the specification to “one example”, “anotherexample”, “an example”, and so forth, means that a particular element(e.g., feature, structure, and/or characteristic) described inconnection with the example is included in at least one exampledescribed herein, and may or may not be present in other examples. Inaddition, it is to be understood that the described elements for anyexample may be combined in any suitable manner in the various examplesunless the context clearly dictates otherwise.

It is to be understood that the ranges provided herein include thestated range and any value or sub-range within the stated range. Forexample, a range from about 55° C. to about 450° C. should beinterpreted to include not only the explicitly recited limits of about55° C. to about 450° C., but also to include individual values, such as57° C., 95° C., 125° C., 250° C., etc., and sub-ranges, such as fromabout 70° C. to about 325° C., from about 60° C. to about 170° C., etc.Furthermore, when “about” is utilized to describe a value, this is meantto encompass minor variations (up to +/−10%) from the stated value.

In describing and claiming the examples disclosed herein, the singularforms “a”, “an”, and “the” include plural referents unless the contextclearly dictates otherwise.

While several examples have been described in detail, it will beapparent that the disclosed examples may be modified. Therefore, theforegoing description is to be considered non-limiting.

What is claimed is:
 1. A coalescing agent for three-dimensional (3D)printing a polymeric build material, the coalescing agent comprising: aco-solvent present in an amount ranging from about 21.5 wt % to about 30wt % of a total wt % of the coalescing agent; a surfactant present in anamount ranging from about 0.5 wt % to about 1.4 wt % of the total wt %of the coalescing agent, the surfactant having a hydrophilic lipophilicbalance (HLB) value that is less than 10; a carbon black pigment presentin an amount ranging from about 3.0 wt % to about 6.0 wt % of the totalwt % of the coalescing agent; a polymeric dispersant having a weightaverage molecular weight ranging from about 12,000 to about 20,000; anda balance of water.
 2. The coalescing agent as defined in claim 1,further comprising a chelator present in an amount ranging from about0.03 wt % to about 0.10 wt % of the total wt % of the coalescing agent.3. The coalescing agent as defined in claim 1 wherein the polymericdispersant is selected from the group consisting of a styrene acrylateand a polyurethane.
 4. The coalescing agent as defined in claim 1wherein the co-solvent has a boiling point of less than 300° C.
 5. Thecoalescing agent as defined in claim 4 wherein the co-solvent isselected from the group consisting of 2-Pyrrolidinone, 1,5-Pentanediol,Triethylene glycol, Tetraethylene glycol, 2-methyl-1,3-propanediol,1,6-Hexanedol,Tripropylene glycol methyl ether, and combinationsthereof, and wherein the coalescing agent excludes other co-solvents. 6.The coalescing agent as defined in claim 1 wherein the surfactant is aself-emulsifiable surfactant based on acetylenic diol chemistry orincludes a combination of a fluorosurfactant and a self-emulsifiablesurfactant based on acetylenic diol chemistry.
 7. The coalescing agentas defined in claim 1, further comprising an anti-kogation agentincluding a combination of a polyacrylic acid polymer and Oleth-3Phosphate, and wherein the combination is present in an amount rangingfrom greater than 0.20 wt % to about 0.62 wt % of the total wt % of thecoalescing agent.
 8. A coalescing agent for three-dimensional (3D)printing a polymeric build material, the coalescing agent consisting of:a co-solvent present in an amount ranging from about 21.5 wt % to about30 wt % of a total wt % of the coalescing agent; a surfactant present inan amount ranging from about 0.5 wt % to about 1.4 wt % of the total wt% of the coalescing agent, the surfactant having a hydrophiliclipophilic balance (HLB) value that is less than 10; an anti-kogationagent present in an amount ranging from greater than 0.20 wt % to about0.62 wt % of the total wt % of the coalescing agent; a carbon blackpigment present in an amount ranging from about 3.0 wt % to about 6.0 wt% of the total wt % of the coalescing agent; a polymeric dispersanthaving a weight average molecular weight ranging from about 12,000 toabout 20,000; a chelator present in an amount ranging from about 0.03 wt% to about 0.10 wt % of the total wt % of the coalescing agent;optionally a biocide; and a balance of water.
 9. The coalescing agent asdefined in claim 8 wherein the biocide is present in the coalescingagent, and wherein the biocide is present in an amount ranging fromabout 0.30 wt % to about 0.40 wt % of the total wt % of the coalescingagent.
 10. A layer of a 3D printed object, comprising: a build material;and a coalescing agent applied on a surface of at least some of thebuild material, the coalescing agent including: a co-solvent present inan amount ranging from about 21.5 wt % to about 30 wt % of a total wt %of the coalescing agent; a surfactant present in an amount ranging fromabout 0.5 wt % to about 1.4 wt % of the total wt % of the coalescingagent, the surfactant having a hydrophilic lipophilic balance (HLB)value that is less than 10; a carbon black pigment present in an amountranging from about 3.0 wt % to about 6.0 wt % of the total wt % of thecoalescing agent; a polymeric dispersant having a weight averagemolecular weight ranging from about 12,000 to about 20,000; and abalance of water.
 11. The layer as defined in claim 10 wherein the buildmaterial is a polymer selected from the group consisting of polyamide,polyethylene, polyethylene terephthalate (PET), polystyrene, polyacetal,polypropylene, polycarbonate, polyester, polyurethanes, and blendsthereof.
 12. The layer as defined in claim 10 wherein the coalescingagent further includes: an anti-kogation agent present in an amountranging from greater than 0.20 wt % to about 0.62 wt % of the total wt %of the coalescing agent; a chelator present in an amount ranging fromabout 0.03 wt % to about 0.10 wt % of the total wt % of the coalescingagent; and a biocide present in an amount ranging from about 0.30 wt %to about 0.40 wt % of the total wt % of the coalescing agent.
 13. Thelayer as defined in claim 10 wherein the co-solvent has a boiling pointof less than 300° C., and wherein the co-solvent is selected from thegroup consisting of 2-Pyrrolidinone, 1,5-Pentanediol, Triethyleneglycol, Tetraethylene glycol, 2-methyl-1,3-propanediol, 1,6-Hexanedol,and Tripropylene glycol methyl ether.
 14. The layer as defined in claim10 wherein the surfactant is a self-emulsifiable surfactant comprisingan acetylenic diol or includes a combination of a fluorosurfactant and aself-emulsifiable surfactant comprising an acetylenic diol.
 15. Thelayer as defined in claim 10 wherein the polymeric dispersant isselected from the group consisting of styrene acrylate and polyurethane.16. The coalescing agent as defined in claim 1 wherein the co-solvent ispresent in an amount of about 25 wt % based on the total wt % of thecoalescing agent.
 17. The coalescing agent as defined in claim 1 whereinthe coalescing agent has a 10-ms dynamic surface tension of 26 dynes/cm.18. The coalescing agent as defined in claim 8 wherein the coalescingagent has a 10-ms dynamic surface tension of 26 dynes/cm.